A process is disclosed for the repair of fan blades of turboengines made of titanium in the publication "State of the Art in Repair of Drive Mechanisms", German Society for Welding Technology, Report Series 85, 1978, pages 72 to 79. In this process, weld filler strips conforming to the cut edge of the blade stump and edge members of weld filler or the material of the weld stump are positioned at the cut edge of the blade and a repair plate that is at least the thickness of the separated and damaged blade region, is positioned adjacent to the cut edge, the facing surface of the repair plate accurately conforming to the weld filler strips and the edge members whereupon an electron beam welding operation is carried out, whereby the weld filler strips and the edge members are welded partially or completely along with the facing surface of the repair plate to the blade stump.
An essential disadvantage of this repair process is that it is unsuitable for the repair of many small and ultrasmall blades, for example, of compressor blades for turbines, due to the expensive preparation of four to five shaped pieces for the repair of one single damaged blade part. For a commercially justifiable repair of structural parts of turboengines, it is necessary that the repair costs remain below approximately 60% of the manufacturing cost of the structural part. This is satisfied in the above process for large size fan blades of titanium with its corresponding high material and finishing costs, but cannot be achieved in the case of small size structural parts.
EPA-0-392,656 discloses that an accurately adapted replacement part which has a profile thickness corresponding to the damaged region for repair of turbine blades, which is then welded to the blade stump with an X seam. One disadvantage of this process is the need for accurate production of the replacement part and a further disadvantage is the requirement for a filler material, which must be introduced into the weld seam.
DIN 65 118, Part 2, page 7, table 3 requires that for a butt weld, the repair plate to be welded on should not exceed an established overdimension of 50%, between the joined structural parts especially in the case of highly stressed parts. Consequently, when welding repair plates to blade stumps, either the repair plate must accurately conform to the contour of the blade stump before welding or it may project beyond the contour up to the maximum given overdimension. The application of this standard is a problem in the region of the leading and trailing edges of the blade where the profile thickness reduces to zero. Here expensive processing methods must take care that the repair plate extending along the contour of the blade stump does not project beyond the permissible overdimension.
If welding is conducted, without overdimensioning, with a repair plate of a profile corresponding to that of the blade stump and without a weld filler, then serious flaws result. These include formation of edge notches at both sides of the weld joint. In the region of the trailing and leading edges of the blade, high instabilities of the formed pool of weld material occur due to the diminution of the thickness of the blade to zero, despite complex and expensive control of the welding parameters as a function of the variation of the profile thickness of the blade stump. This may lead to sagging of the weld pool and to separation of material from the weld pool. Under certain conditions, the weld pool must be prevented at the edges from sagging and separation by support from flow by shaped mold parts. This is a great disadvantage. The filler material, which fills the edge notches forms a raised weld bead which must subsequently be machined in order to obtain a smooth, continuous transition between the profile of the blade stump and that of the profiled repair plate.
A further disadvantage in the butt welding of profiled repair plate, which conform to the profile of the blade stump, is the unavoidable distortion of both the blade stump and the profiled repair plate in the case of blades of small thickness, particularly compressor blades. In addition to machining off the weld bead an expensive realignment of the repaired blade is necessary.